As explained in Section 3.1.3, Gaia data
processing uses the rigorous relativistic definitions of reference
systems including time scales as their integral parts. The coordinate
time of BCRS — TCB (Soffel et al. 2003) — is used throughout
data processing and parametrizes the final Gaia catalogue.
Another important technical time coordinate used in Gaia data
processing is the On-Board Mission Timeline (OBMT). OBMT represents
the readings of the on-board Gaia atomic clock plus a constant chosen
for each continuous time interval between Gaia clock resets in such a
way that OBMT remains increasing with physical time. Strictly speaking,
OBMT is not a time scale since it is not necessarily continuous. OBMT
is a purely technical time coordinate that is however unique for any
event on board of Gaia. Although the raw observations of Gaia are
parametrized by OBMT, for various purposes (e.g. interrogating the
solar-system and Gaia ephemerides), OBMT should be related to
TCB. This is done by creating the Gaia time ephemeris — a model of
Gaia’s clock fitted to the special time synchronization data — the
one-way time transfer from Gaia to the ESA
ground stations (Klioner 2015; Klioner et al. 2017).